1. Field of the Invention
The present invention relates to a fluid bearing and a brushless motor having the same, and more particularly to a fluid bearing having an oil circulating mechanism for generating kinetic pressure therein, and a brushless motor having the fluid bearing.
2. Description of the Related Art
Recently, a data recording/reproducing device, such as a disk media, etc., is required to be able to operate in high speed. Therefore, to meet this requirement, a kinetic pressure fluid bearing is widely used as a bearing for a spindle motor which is used in the data recording/reproducing device.
In the kinetic pressure fluid bearing, kinetic pressure generating grooves such as herring bone grooves are formed on the outer periphery of a rotating shaft or on the inner periphery of a bearing sleeve at sliding contact portion where the rotating shaft comes in contact with the bearing sleeve. The gap defined at the sliding contact portion between the rotating shaft and the bearing sleeve is filled with lubricant. As the rotating shaft rotates, a pressure is applied to the lubricant in the gap, which causes the rotating shaft of a rotor to be rotatably supported.
The lubricant, which is used in the kinetic pressure fluid bearing, is expanded by the friction heat generated during the rotation of the rotating shaft, which is leaked out of the gap. In this case in which the lubricant is exhausted due to the leakage in the gap, the efficiency of the bearing is remarkably deteriorated. Therefore, it is necessary that a lubricant reservoir should be disposed near the sliding contact portion. Thereby, the lubricant is temporarily reserved in the lubricant reservoir when the lubricant is sufficiently supplied to the gap, while the lubricant is supplied into the gap from the lubricant reservoir when the lubricant is insufficient in the gap.
The gap of the sliding contact portion is preferably about 5˜20 μm. In general, when a plurality of the sliding contact portions having the gap is formed on one rotating shaft, the lubricant reservoir is installed in each sliding contact portion. When the gap is filled with the lubricant, each space in the sliding contact portions is nearly sealed. When a pressure difference exists between the lubricant reservoirs which are blocked by the respective sliding contact portion, this causes the lubricant to be unevenly supplied to the respective sliding contact portion. Therefore, there is a problem in that it is difficult to generate the kinetic pressure uniformly. To solve this problem, the lubricant reservoirs are connected with one another so that the pressure in the respective lubricant reservoir is constant, while the bearing is also provided with a hole for circulating the lubricant therein.
In the kinetic pressure fluid bearing as constructed above, the bearing itself can be made in small size. Further, since a fluid, for example lubricant, presents between the rotating shaft and the bearing, the bearing is excellently operated in low noise and in low vibration, and has strong shock-endurance. Further, the bearing has a great load allowance, so that it is possible to significantly prevent the rotating shaft from being shaken.
In the kinetic pressure fluid bearing as described above, for example, the oil circulating hole can be formed in the bearing which is comprised of a cylindrical bearing sleeve and a housing receiving the bearing sleeve. It is known that an axial concave groove as the oil circulating hole is formed on the outer peripheral surface of the bearing sleeve or on the inner peripheral surface of the housing to which the bearing sleeve is fixedly attached. Further, it is well known that a radial concave groove or a radial traverse hole can be formed on the end surface of the bearing sleeve, or that a through-hole can be formed in the rotating shaft to pass from the end surface of the rotating shaft disposed in the bearing to the outer peripheral surface near a kinetic generating portion. In these structures, the total number of parts is increased, and the structures become complicated. Further, forming the concave groove or the through-hole requires numerous man-hours and high precision.